(1) Field of the Invention
The present invention relates generally to the field of time-domain reflectometers. More specifically, it relates to such reflectometers which are a part of a photonic system application in which the object of the reflectometry is a span of fiber which has an interrogation signal launch end and a remote end. The invention enables the provision of a linear array of virtual sensors along the span. One particular type of application toward which the invention is directed are acoustic security alarm systems in which the span serves as a perimeter intrusion monitoring line.
(2) Description of the Prior Art
The U.S. Department of the Navy has been engaged in the development of towed acoustic arrays which are reflectometric systems in which the object of the reflectometry is a fiber span having an interrogation signal launch end and a remote end. One such development involves forming a towed array of acoustic sensors along the span by the costly process of irradiating Bragg reflective gratings into the fiber cable. These reflective gratings form the array of sensors of the reflectometry scheme of these systems. These towed arrays have a length of the order of at least 1.0 km, and the need to irradiate the fiber has resulted in the fiber spans costing hundreds of thousands of dollars each.
The Department of the Navy development activities have been further tasked to apply their creative efforts to homeland defense problems. As part of this effort there is under consideration the use of a reflectometer in which a fiber span is the object of the reflectometry. In this scheme, the fiber span provided with acoustic sensors would be used as an intrusion detector to monitor the perimeter of an area desired to be secure. The span lengths for this type of application include lengths of the order of 5 km, (links of a U.S. border protection network, oil line protection, chemical plant protection, etc.). In such perimeter monitoring applications thousands of acoustic sensors would be required along the fiber span.
The cost of manufacturing such perimeter monitoring spans employing reflective Bragg grating sensors has been an obstacle to their use in perimeter intrusion monitoring applications. Thus, there is considerable interest in the development of a reflectometer system in which a fiber span is the object of the reflectometry optic array that does not require the high cost of Bragg reflective acoustic sensors.
Previous effort in solving related problems are described by the following patents:
U.S. Pat. No. 5,194,847 issued Mar. 16, 1993 to H. Taylor and C. Lee discloses an apparatus for sensing intrusion into a predefined perimeter which comprises means for producing a coherent pulsed light, which is injected into an optical sensing fiber having a first predetermined length and positioned along the predefined perimeter. A backscattered light in response to receiving the coherent light pulses is produced and coupled into an optical receiving fiber. The backscattered light is detected by a photodetector and a signal indicative of the backscattered light is produced. An intrusion is detectable from the produced signal as indicated by a change in the backscattered light. To increase the sensitivity of the apparatus, a reference fiber and interferometer may also be employed.
U.S. Pat. No. 6,285,806 issued on Sep. 4, 2001 to A. Kersey et al., discloses an apparatus and method for measuring strain in an optical fiber using the spectral shift of Rayleigh scattered light. The interference pattern produced by an air gap reflector and backscatter radiation is measured. Using Fourier Transforms, the spectrum of any section of fiber can be extracted. Cross correlation with an unstrained measurement produces a correlation peak. The location of the correlation peak indicates the strain level in the selected portion of optical fiber.
The above patents do not show how to obtain signals representing acoustic pressure signals incident upon a fiber span (to detect perimeter intrusion) at a very large number of sensing stations without involving high manufacturing costs. Consequently, those skilled in the arts will appreciate the present invention which addresses these and other problems.